These days, several detection and analysis methods for determining physiological parameters in bodily-fluid samples or other parameters in other biological samples are carried out in an automated fashion in great numbers in corresponding automated analysis instruments. In the field of laboratory diagnostics, the bodily-fluid samples to be analyzed, such as blood, plasma, serum or urine, are provided in sealed sample vessels. The sample vessels are supplied to the analysis instrument individually or in groups, arranged in suitable frames. The sample vessels are usually, by means of a transport system, first of all routed past an identification apparatus, which reads information applied to the sample vessel, e.g. in the form of a barcode, in respect of the identity of the sample and transmits that information to a storage unit. Then, an aliquot of the sample liquid is withdrawn from each sample vessel and transferred into a reaction vessel, in which the analytic test method is then carried out.
Bodily-fluid samples are usually situated in sample vessels made of polymers, less commonly of glass, which are sealed by a cover or a cap made of polymer or by a rubber plug with a thickness of up to 1 cm. Blood, plasma and serum samples are preferably supplied to the analysis instrument in the blood withdrawal tubule. Blood withdrawal tubules usually consist of a transparent polymer and the sealing device is equipped with a specific connector for cannulas. Here, except for in the case of the so-called Sarstedt principle, blood withdrawal tubules are often designed as negative-pressure systems: negative pressure prevails within the sample vessel of this type from the outset. If it is plugged onto the adapter connected to the puncturing cannula, blood is suctioned as a result of this negative pressure. An advantage of this system is that the suctioned up amount of blood is comparatively constant and hence it is also possible to measure out precisely the amount of an anticoagulant (e.g. citrate, EDTA, heparin) introduced into the blood withdrawal tubule in advance. The blood withdrawal tubules are usually sealed by an elastic seal for maintaining the pressure.
In order to withdraw sample liquid from the sample vessels and in order to transfer sample liquid into a reaction vessel, an automated analysis instrument comprises a sample pipettor with a hollow needle. The hollow needle is attached to a transport arm and can thus be moved between at least one sampling position and at least one sample delivery position. At the sampling position, the hollow needle is moved vertically downward, where possible along the central axis of the sample vessel, until the needle tip is immersed in the sample liquid. The immersion is registered with the aid of an appropriate sensor. By generating negative pressure in the hollow needle, sample liquid is suctioned in; the hollow needle is moved vertically upward and subsequently moved horizontally to the sample delivery position. At the sample delivery position, a defined amount of sample is then placed into a reaction vessel. Known hollow needles for such sample pipettors often consist of stainless steel and have a substantially cylindrical basic shape with a central hollow channel, wherein the hollow needle can have axial portions with varying internal and external radii.
If the sample is to be withdrawn from sealed sample vessels, the sample pipettor with hollow needle must be designed in such a way that the vertical downward movement of the hollow needle is carried out with such a force that the sealing device of the sample vessel can be pierced. However, it must be ensured at the same time that the hollow needle is not damaged because there could otherwise be errors during sampling or sample delivery.
In order to keep the amount of force applied for piercing a sealing device of a sample vessel as low as possible, hollow needles provided for this have a comparatively solid design and are usually sharpened. EP-B1-1420255 (FIGS. 34-37; paragraphs 0111-0118) for example describes a hollow needle which has a pyramidal or conical shape at the tip such that an apex is created, at which the force during the downward movement of the hollow needle is focused and a sealing device of a sample vessel, e.g. a rubber cap, can be pierced with comparatively little force being applied.
A further problem when piercing sealing devices with the hollow needle of a sample pipettor consists of the punching-blade effect of the needle, which leads to parts of the perforated sealing device, e.g. rubber crumbs, possibly plugging the pipetting hole of the needle. This problem is solved, inter alia, in DE-T2-69827465 (U.S. Pat. No. 6,135,172) by virtue of the fact that the hollow channel does not open up at the tip itself, but rather laterally on the needle body.
A further problem consists of errors during the sampling being created, in particular, by virtue of the fact that the needle is deflected laterally upon contact with the sealing element of a sample vessel despite careful adjustment and does not pierce along or at least parallel to the central axis of the sample vessel as desired, but rather pierces through the sealing element at an angle. In the worst case, this can lead to the needle touching the inner wall of the sample tubule and possibly even destroying the latter, as a result of which the sample and/or the needle can become unusable and the instrument could possibly be contaminated. Furthermore, it was observed that the sensor signal for the immersion into the sample may be triggered when the needle contacts the inner wall of the sample vessel, even though actual immersion has not yet taken place. This increases the risk of air being pipetted instead of sample liquid.